export const getUUID =  (len = 20)=> { // len 长度 radix 基数
    var chars = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz'.split('')
    var uuid = []; var i
    const radix = chars.length
    if (len) {
        // 紧凑型
        for (i = 0; i < len; i++) uuid[i] = chars[0 | Math.random() * radix]
    } else {
        // rfc4122, version 4 form
        var r
        // rfc4122 requires these characters
        uuid[8] = uuid[13] = uuid[18] = uuid[23] = '-'
        uuid[14] = '4'
        // 填写随机数据  在i==19处，将时钟序列的高位设置为
        // per rfc4122, sec. 4.1.5
        for (i = 0; i < 36; i++) {
            if (!uuid[i]) {
                r = 0 | Math.random() * 16
                uuid[i] = chars[(i === 19) ? (r & 0x3) | 0x8 : r]
            }
        }
    }
    return uuid.join('')
}
